There is a growing need for higher power microwave applications of diodes such as for solid state switches, oscillators, phase shifters, and amplifiers in radar and communication systems. To meet these needs the power handling capacity of the diode packages must be increased considerably.
Device temperature is the ultimate limiting factor in power handling capability. Device temperature has been lowered in a number of ways, but, in general, designs are pursued which provide lower thermal resistance in the device package and circuits are utilized which improve electrical matching, and provide lower series electrical resistance. These circuits have the desired effect of reducing heat energy input to the device while the designs which provide lower thermal resistance in the device package are used to remove energy after it is generated.
Presently available package configurations include the conventional ceramic pill package for the PIN diode. Above 500 MHz the parasitic electrical elements of this package usually limit the power handling capability of the diode by increasing the diode power losses. These parasitic elements also restrict the usable bandwidth. These two problems make parasitic reduction packages of great interest.
Parasitic reduction packages are available from many sources. Seals are typically made with Kovar leads through glass. Kovar is advantageous because its coefficient of thermal expansion closely matches that of glass and it forms an oxide that joins in a hermetic seal with the glass. Unfortunately, this circular cross section Kovar wire has a relatively high electrical resistance and the material is magnetic. The magnetic permeability of the material reduces the depth of penetration of the RF current and hence reduces the effective cross section through which the current flows. The resulting RF resistance for the Kovar wire is so high that it causes the leads to heat to the melting point when diodes in this type of package are tested as 200 watt switches at a frequency of approximately 3 GHz.
For gigahertz frequencies, a parasitic reduction package is needed that is useful for high power diode applications such as switching 600 watts or more.
Additionally, circuits integral to the device package must provide good impedance matching with the external circuits to provide efficient transfer of RF energy with minimized losses.